Recently, radio communication systems have been developed which employ transmitters with variable output power. Variable power systems are advantageous in systems with a large number of channels and users. One example of this type of system currently being implemented in the United States. In this type of system, a geographic area is divided into a number of "cells" wherein users in any particular cell may be assigned any of a number of frequencies. As a particular user moves closer or further away from a particular cell controller, the cell controller reduces or increases the power of the mobile transmitter. The output power of the mobile transmitter is maintained at whatever level is required to provide a satisfactory communication channel. As the power level of a particular mobile transmitter operating on a given frequency in a particular cell is reduced, the power level of a particular mobile transmitter operating on the same frequency in an adjacent cell may be increased, and vice versa. This allows the various operating frequencies of adjacent cells to be used with optimum efficiency.
Several techniques have been developed in the past for controlling the output power of an RF power amplifier. One example is disclosed in U.S. Pat. No. 4,523,155, invented by Walczak, et al. and issued Jun. 11, 1985. The circuit of Walczak, et al. varies the output power of an RF power amplifier in response to a plurality of control signals referred to as Automatic Output Control (AOC) signals. This invention further includes temperature compensation means so that the circuit can be predictably operated over a wide range of temperatures. While this invention is presumably useful, it is quite complex, and it requires a number of additional components to provide the temperature compensation feature. Whenever additional components are incorporated in an electronic circuit, its reliability is necessarily reduced.
Another technique for controlling the output power of an RF amplifier is disclosed in U.S. Pat. No. 4,602,218, invented by Vilmur, et al. and issued Jul. 22, 1986. The circuit invented by Vilmur, et al. also provides a plurality of output power levels in response to a plurality of AOC signals and it is designed to operate in the same type of system as the circuit of Walczak et al. Both the Walczak, et al. and Vilmur, et al. inventions employ a diode rectifier to derive a feedback signal for controlling the power level of the RF amplifier output stage. While the circuit of Vilmur, et al. is apparently less complex than the circuit of Walczak, et al., it provides an improvement over the Walczak invention by compressing the dynamic range of the feedback signal prior to the signal's application to the diode rectifier. According to Vilmur, et al., this dynamic range compression eliminates the need for specific temperature compensation circuits. However, the circuit of Vilmur, et al. employs a logarithmic amplified with a non-linear gain function to achieve this result. While the circuit of Vilmur, et al. may achieve its stated purpose, the performance of non-linear systems are difficult to predict under all conditions and maybe prone to instability in many instances. Furthermore, the circuit invented by Vilmur, et al. is also relatively complex, requiring a compression or dynamic range reduction circuit to achieve its stated purpose. Each of the references mentioned above is incorporated herein by reference.